The Facebook CEO sounded off during a Facebook Live chat.

Elon Musk and Mark Zuckerberg have some major things in common: They're both CEOs of prominent Silicon Valley companies, and both of those companies rely heavily on artificial intelligence.

But the two evidently diverge when it comes to their opinions on the technology. Musk has become an increasingly vocal critic of A.I. in recent years, going so far as to call it "the biggest threat we face as a civilization" earlier this month. During a Facebook Live chat on Sunday, a viewer asked Zuckerberg what he thought about the future of A.I., citing Musk's comments. Facebook's CEO didn't hold back, though he refrained from calling out Musk by name.

"I think people who are naysayers and try to drum up these doomsday scenarios--I don't understand it," Zuckerberg said. "It's really negative, and in some ways I think it's pretty irresponsible."

Zuckerberg cited some of the ways A.I. is already improving lives, like aiding in the diagnoses of illnesses and assisting in drug discovery. In general terms, he brushed off the criticisms people have about the technology.

"I have pretty strong opinions on this. I'm really optimistic," Zuckerberg said from his backyard while waiting for his dinner to slow cook. "Technology can always be used for good and bad, and you need to be careful about how you build it, and what you build, and how it's going to be used. But people are arguing for slowing down the process of building A.I.--I just find that really questionable. I have a hard time wrapping my head around that."

In his defense of A.I., Zuckerberg specifically called out self-driving cars. "One of the top causes of death for people is car accidents still," Zuckerberg said. "If you can eliminate that with A.I., that is going to be just a dramatic improvement in people's lives." Tesla's vehicles already can assist drivers with highway driving, and the company has said its goal is for its cars to be fully autonomous by the end of this year.

Earlier this month, Musk cited his closeness to A.I. as a reason for his fears--and a reason that others should find them credible. "I have exposure to the most cutting edge A.I., and I think people should be really concerned by it," he said. "A.I. is a rare case where I think we need to be proactive in regulation [rather] than be reactive."

Last month, in response to a survey that revealed experts believe A.I. will surpass human skills on all tasks by 2060, Musk tweeted that he believed it would be more like 2030 to 2040. "I hope I'm wrong," he added.

Zuckerberg's Facebook has relied more and more on A.I. in recent years. Its software is capable of identifying the objects in images, producing captions for the visually impaired. Its Messenger service can detect when people are in need of services, such as a taxi ride or a marketplace listing, by scanning their written communications. Last month, the company announced that it's training chatbots to negotiate--and that they're already just as good as humans.

"If you're arguing against A.I., then you're arguing against safer cars and being able to better diagnose people when they're sick," Zuckerberg said Sunday. "I just don't see how in good conscience some people can do that. So I'm just much more optimistic on this in general than I think a lot of folks are."